Antihistamines might help some for dogs (and cats) with atopy (skin, paw and ear problems due to airborne allergens). They are less useful for treating contact, flea, or food allergies. While antihistamines alone are not going to completely eliminate symptoms, they may decrease severity and reduce dependency on cortisones (steroids) and/or cyclosporine.
The positives to antihistamines: Antihistamines are good choice for owners who want to avoid cortisones and are unable to afford cyclosporine or desensitization
The downsides to antihistamines: Antihistamines are not a cure for allergies. Like cortisones and cyclosporine, they only suppress allergic reactions. They are only effective in up to 33% of dogs, and up to 73% of cats with atopy. They are not going to eliminate all itchiness and the need for other treatments.
Just like humans, an animal may be responsive to one antihistamine but not another. Normally a 3-6 week cycle of antihistamine experimentation is required. If you’re going to try one, be sure you do it during the times of year when your pet’s allergies are known to be unrelentingly bad. If you start treating with a drug and coincidentally the plant that makes your pet itch (for example) stops flowering, you may be lead to believe the antihistamine worked well. Spring and summer are usually the worst allergy seasons. To know that the improvement is really a drug effect, it is best to re-challenge; stop the antihistamine after a month of use, and watch for the symptoms to return. If they do, you win; if not, you need to begin a trial with another antihistamine. For dogs, I like to use doxepin or pentoxyfylline. I’m considering trying Zyrtec or Claritin. For cats, I like to use chlorpheniramine. Ask your veterinarian for a dosage for your pet.
Another tip that you might want to try is putting your dog on an Essential Fatty Acid Supplement or fish-based diet, as such diets seem to make antihistamines more potently effective.
There are some side effects but they are uncommon and will usually resolve when you discontinue treatment:
- Drowsiness: may dissipate after 3-7 days of use, so do persist if the side effect is not too severe. Speak with your vet before giving antihistamines to dogs on anti-seizure medication as they may increase sedative side effects.
- Stimulatory: excitement, vocalizing, hunger, or thirst.
- Gastrointestinal: salivation, vomiting, diarrhea, decreased appetite.
- Hematologic: side effects are rare and detected by blood test if using some agents in the longer-term.
- Increased blood pressure: so use with caution in animals with heart disease, hypertension, glaucoma and hyperthyroidism.
Notes on diphenhydramine (Benadryl): Be careful not to confuse Benadryl Original Formula, with other Benadryl combination products. Some of the other Benadryl products contain Tylenol or Sudafed which are toxic to pets.